1. Field of the Invention
The present invention relates generally to fuel assemblies for nuclear reactors and, more particularly, is concerned with apparatus and method for preassembling a top nozzle as a completed subassembly for later application to the skeleton of the fuel assembly.
2. Description of the Prior Art
In most nuclear reactors the core portion is comprised of a large number of elongated fuel elements or rods grouped in and supported by frameworks referred to as fuel assemblies. The fuel assemblies are generally elongated and receive support and alignment from upper and lower transversely extending core support plates. These upper and lower core support plates are directly or indirectly attached to a support barrel which surrounds the entire core and extends between the ends thereof. In the most common configuration, the axis of the core support barrel extends vertically and the various fuel assemblies are also arranged vertically resting on the lower support plate.
Temperatures at various times within the core may vary greatly, such as, from cold shutdown to normal operating conditions. It is also a well-known fact that different materials exhibit different thermal growth characteristics. Therefore, since the materials used in the vertically extending support structures of the fuel assemblies are generally different than those used in the core support barrel, the thermal expansion of these various members in the axial or vertical direction may be quite significant, particularly, at the high temperatures found within the core and the axial length of some of the members. For these reasons, the fuel assemblies are not usually attached to the upper and lower core plates but rather are supported in a manner which permits some relative motion therebetween. The axial thermal expansion differential between the fuel assemblies and the core support barrel has been accommodated by ensuring that the axial spacing between the upper and lower core support plates is somewhat greater than the axial length of the fuel assemblies. Normally, this is accomplished by providing an axial gap or space between the top of the fuel assemblies and the upper core support plate.
Further, generally in most reactors, a fluid coolant, such as water, is directed upwardly through apertures in the lower core support plate and along the fuel rods of the various fuel assemblies to receive the thermal energy therefrom. The physical configuration of the fuel assemblies is such that the coolant may experience a significant pressure drop in passing upwardly through the core region. This pressure drop necessarily produces a lifting force on the fuel assemblies. In some instances, the weight of the fuel assembly is sufficient to over come the upward hydraulic lifting forces under all operating conditions; however, this is often not the case, particularly when the coolant density is high, as at reactor start-up, and additionally because of increasing coolant flow rates. When the hydraulic forces in the upward direction on a particular fuel assembly are greateer than the weight of that fuel assembly, the net resultant force on the fuel assembly will be in the upward direction, causing the assembly to move upward into contact with the upper core plate. This upward motion of the fuel assembly, if uncontrolled, may result in damage to the fuel assembly and the fuel rods or to the upper core plate and must, therefore, be avoided.
Conventional designs of fuel assemblies include a multiplicity of fuel rods and control rod guide thimbles held in an organized array by grids spaced along the fuel assembly length. The grids are attached to the control rod guide thimbles. Top and bottom nozzles on opposite ends thereof are secured to the guide thimbles which extend above and below the ends of the fuel rods. At the top end of the assembly, the guide thimbles are attached in openings provided in the top nozzle. Conventional fuel assemblies also have employed a fuel assembly hold-down device to prevent the force of the upward coolant flow from lifting a fuel assembly into damaging contact with the upper core support plate of the reactor, while allowing for changes in the fuel assembly length due to core induced thermal expansion and the like. Representative of such hold-down devices are the ones described and illustrated in U.S. Patents to Andrews et al (No. 3,379,619), Klumb et al (No. 3,770,583) and Anthony (No. 4,192,719).
One recent fuel assembly design having a top nozzle subassembly which incorporates hold-down means is described and illustrated in the patent applications cross-referenced above. The top nozzle subassembly with its hold-down means basically includes an upper bearing plate, a lower adapter plate, a plurality of guide thimble extension tubes extending between and through the plates, and a plurality of coil springs encircling the extension tubes and held in a state of compression between the plates by a pair of retainers on the extension tubes. The lower retainer is a collar attached to each extension tube below the lower adapter plate so as to limit downward sliding movement of the adapter plate along the extension tube. The upper retainer is attached on the upper end of each extension tube within a passageway in the upper bearing plate and cooperates with an internal ledge therein to limit upward movement of the bearing plate along the extension tube. The construction of the subassembly is such that the preloaded coil springs prevent upward lifting of the fuel assembly while the terminal upper end of each extension tube along with the upper retainer are permitted to reciprocate within a respective passageway of the upper bearing plate, thus allowing for thermal growth of the guide thimbles of the fuel assembly to which the extension tubes are attached when the top nozzle subassembly is applied to the fuel assembly.
While the top nozzle subassembly construction briefly described above has demonstrated its ability to provide sufficient hold-down force to prevent hydraulic lifting of the fuel assembly while allowing for changes in fuel assembly length due to core induced thermal expansion, the subassembly does contain a large number of components which must be precisely assembled together so that the subassembly will function properly when applied to the guide thimbles of the fuel assembly. Consequently, a need exists for a technique to preassemble the above-described top nozzle components into a completed subassembly which will provide the desired degree of precision in the placement of the components at their relative positions in the subassembly and compensate for normal tolerance mismatch.